Priority is claimed from Japanese Patent Application No. 2003-316513 filed Sep. 9, 2003, and Japanese Patent Application No. 2003-342174, filed Sep. 30, 2003, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to graded-index multimode fibers and to a manufacturing method therefor. More particularly, the present invention relates to graded-index multimode fibers which have a wide transmission bandwidth in a region around the center wavelength and to a manufacturing method therefor.
2. Description of Related Art
Graded-index multimode fibers (hereinafter referred to as “GI multimode fibers”), which are one type of multimode fiber, have a large numerical aperture and have been used for transmission lines of optical local area networks (LAN). Driven by a need for a faster optical LAN, techniques to precisely control refractive index profiles of GI multimode fibers have been improved.
However, further improvement in performance of GI multimode fibers seems almost impossible at present, and wavelength division multiplexing (WDM) is required in order to increase transmission bandwidth of GI multimode fibers.
In a conventional GI multimode fiber which has a germanium-containing core, however, an optimum refractive index profile varies greatly depending on the wavelength of the light signal propagating through the fiber (see, e.g., R. Olshansky, “Propagation in glass optical waveguides”, Review of Modern Physics, Vol. 51, No. 2, April 1979). Accordingly, since a fiber having a refractive index profile optimized at a certain wavelength provides a very small transmission bandwidth at other wavelengths, it cannot be used for wavelength division multiplexing (WDM).
In order to achieve a wide transmission bandwidth and minimize differences in propagation speed among different modes, GI fibers are fabricated so that profiles (refractive index profiles) of these optical fibers have a graded-index profile. Such GI multimode fibers have a larger numerical aperture, and have been widely used in various applications, e.g., the transmission lines of optical LANs.
In general, the refractive index profile of GI fibers can be defined by the following Formula I:
                              n          ⁡                      (            r            )                          =                  {                                                                                                                n                      1                                        ⁡                                          [                                              1                        -                                                  F                          ⁡                                                      (                            r                            )                                                                                              ]                                                                            1                    /                    2                                                                                                                  ⁢                                      (                                          0                      ≤                      r                      ≤                      a                                        )                                                                                                                                                                  n                      1                                        ⁡                                          [                                              1                        -                                                  F                          ⁡                                                      (                            a                            )                                                                                              ]                                                                            1                    /                    2                                                                                                                  ⁢                                      (                                          r                      >                      a                                        )                                                                                                          (        I        )            where F is a function representing the shape of the profile, “a” is a core radius, n, is the refractive index at the center of the core, and “r” is a distance between any given position within the core and the center of the core.
Conventional GI fibers are essentially made of silica glass and are doped with an amount of one or more dopants, e.g., germanium (Ge). The concentration of the one or more dopants, e.g., germanium, is varied in the cross-sectional direction.
When a single dopant is used, the shape of the profile F of the dopant can be defined by the following Formula II:
                              F          ⁡                      (            r            )                          =                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      α                                              (        II        )            here Δ is the relative refractive index difference at the center of the core with respect to the cladding, and α is a refractive index profile exponential parameter.
Since the optimum value αopt which provides a high transmission bandwidth is wavelength dependent, a maximum transmission bandwidth is obtained only at a certain wavelength. Conventional GI fibers which are doped with a single dopant (e.g., germanium) are optimized for a certain wavelength since an optimal shape of the profile of such fibers varies depending on wavelength.
Profiles of conventional GI multimode fibers doped with a single dopant, e.g., germanium (Ge) are controlled very precisely, and further improvement in performance seems almost impossible at present. Recently, demands for faster optical LANs have created a need for multimode fibers, e.g., GI fibers having a wider transmission bandwidth than those currently available. Thus, the use of wavelength division multiplexing (WDM) has been studied.
However, since GI multimode fibers doped with a single dopant, e.g., germanium (Ge), are optimized for a certain wavelength as mentioned above, the transmission bandwidth of such fibers becomes significantly small at wavelengths other than the optimal wavelength. Such fibers are, therefore, unsuitable for wavelength division multiplexing.